Histamine release from mast cells and basophils is being studied as an immunological mechanism involved in inflammation. The activation of these cells by immune cell surface receptors is also a model for signal transduction and cell secretion. Besides cell surface receptors, the cells can be activated by other secretagogues such as the calcium ionophore A23187. The emphasis the last few years has been in understanding the role of protein tyrosine phosphorylations in the signalling. The cultured rat basophilic leukemia cells are used as one of the main models for these studies.